Solute transport through preferential pathways in municipal solid waste

We modelled lithium transport through an undisturbed solid waste sample and a pilot-scale experimental landfill by use of the same probabilistic Lagrangian approach. The waste sample contained 3.5 m3 of old, well-degraded waste, and the experimental landfill 545 m3 of fresh municipal waste. In the waste sample, four tracer tests were performed under either constant water head or sprinkling input boundary conditions, and either steady state or transient flow. All experimental breakthrough curves (BTCs) exhibit greater spreading than can be explained by a local dispersion mechanism, even when combined with diffusive mass transfer between mobile and immobile water. The observed non-uniform flow and transport could only be represented meaningfully by a two-domain conceptual model, assuming mobile water and advective solute transport both through preferential flow paths and in zones of slow flow. In the undisturbed waste sample, the preferential flow quantification implies that 55–70% of the total infiltrated water moves through only 5–16% of the total water content. In the landfill, it is 90% of the vertically flowing water that moves preferentially through 47% of the total water content. The difference between the landfill and the waste sample appears to be related to waste material properties (which may vary both between and within different waste systems) rather than to the prevailing flow conditions (average flow rate or degree of saturation).